01.09.1 Skin

Topical administration of drugs to the skin usually uses patches or ointments, which are applied to the skin. The outer layer of the skin (the epidermis) is a lipid barrier, and aqueous solutions will only be absorbed slowly. If necessary, absorption can be enhanced by suspending the drug in an oily substance and rubbing the resulting preparation into the skin. The epidermis is the main determinant of the rate of absorption from the skin, as the inner layer of the skin (the dermis) is freely permeable. Avoidance of first pass liver metabolism is a major advantage with administration. Thus, drugs that undergo extensive first pass liver metabolism can be used topically. For instance, the nitrates can be given in patches that provide long term release and long term prevention of angina. The nitrates have their effect on blood vessels, and thus require absorption into the systemic circulation to be effective.

Scopalamine patches are another example of tradsdermal patches. Transdermal scopolamine patches are used to prevent motion sickness.

Topical administration can be also be used to produce local effects and avoid systemic effects/side effect, when this is required. The anti-inflammatory glucocorticoids can have severe adverse effects if they get into the blood stream. However, they can be used safely in skin diseases with local application. Topical administration to the skin provides the highest concentrations of glucocorticoids, and the majority of their effects at the required site (the skin). Only low concentrations of the glucocorticoids will enter the systemic circulation with the potential to cause adverse effects.

Controlled-release preparations for topical administration to the skin are increasing being used to give a long term steady state level of drug. For instance, nicotine patches are controlled-release preparations that help to reduce cravings in smoking cessation. Another example is the scopolamine patche, which is a slow release preparations to prevent motion sickness.